The aromatic polyphosphites of this invention are useful as organic polymer stabilizers. They are especially useful for such purpose in olefin polymer compositions, which require a high degree of thermal stability. One of the advantages of polypropylene, for example, is the fact that it is readily processed and fabricated in all of the conventional systems; these include solid and foam molding, solid and composite extrusion, spinning and orienting, rotocasting, powder coating, thermoforming and pressure forming, stamping and laminating. Moreover, post-fabrication operations which are commonly applied include machining, welding, turning, sawing, drilling, butt and spin welding, and hot stamping. Many of these operations are carried out at relatively high temperatures, however, and it is necessary to protect the polypropylene from thermal degradation at these temperatures.
Polypropylene is especially vulnerable to degradation under conditions which favor oxidation, for example, because of the recurring presence in the molecular chain of tertiary carbon atoms. These are notoriously susceptible to oxidative attack.
The problem of oxidative degradation generally is met by incorporation into the olefin polymer composition of a small proportion of a high molecular weight antioxidant. Phenolic phosphite antioxidants are well known and, in most instances, are quite suitable for this purpose. They frequently suffer one disadvantage, however, because of the invariable contamination of the antioxidant with a small proportion of phenol. Such contamination tends to render the polypropylene unsuitable for uses involving probable contact with food, medicines and the like. The contamination arises from the fact that the high molecular weight phenolic phosphite compounds are prepared by a transesterification reaction involving triphenyl phosphite or a phosphorus ester prepared from triphenyl phosphite, and a bisphenol. The product which results from such a reaction contains phenol as a by-product, and it is not practical to remove all of this phenol from the desired product.
U.S. Pat. No. 3,053,878 (Friedman et al.) shows the reaction of diphenyl pentaerythritol diphosphite with bisphenol A in the presence of a diphenyl phosphite catalyst to form a polymeric phosphite.
U.S. Pat. No. 3,305,608 (Baranauckas et al.) shows the reaction of stoichiometric quantities of triphenyl phosphite, pentaerythritol and 4,4-isopropylidenediphenol (bisphenol A) in the presence of a sodium catalyst.
Japanese Patent Publication No. 1975-35097 shows the reaction of triphenyl phosphite and pentaerythritol to form an intermediate product which then is reacted with a mixture of more pentaerythritol and bisphenol A. Stoichiometric quantities are used. The resulting product is said to be effective as a polymer stabilizer.
U.K. Pat. No. 2,041,938 shows a process for preparing aromatic polyphosphites which involves the reaction of dichloro pentaerythritol diphosphite with certain bis-phenolic compounds including 4,4'-butylidene-bis-(6-tertiarybutyl-m-cresol) and bis-(4-hydroxy-2-methyl-5-tertiarybutylphenyl) sulfide. The product of such process is an oligomer.